Marine creatures like crabs have 12 legs

I know marine creatures like crabs have 12 legs, 10 are small and 2 are very big. Do you know why they have small mouths ? do you know what food is for crabs ?
As a side note, I really wonder why human refuse under-water life and choose land life ? I guess perhaps because in water we hold our breath long, right ?

The reason we couldn't live underwater is because our lungs perform resperation wiht the oxygen in the air around us. Underwater creatures like fish, have other organs like gills that can get the oxygen from the water.

The reason we couldn't live underwater is because our lungs perform resperation wiht the oxygen in the air around us. Underwater creatures like fish, have other organs like gills that can get the oxygen from the water.

Then how do you explain fish that *do* breath air *and* oxygen in water, i.e. some specialized fishes in the mangroves?

I know marine creatures like crabs have 12 legs, 10 are small and 2 are very big. Do you know why they have small mouths ? do you know what food is for crabs ?
As a side note, I really wonder why human refuse under-water life and choose land life ? I guess perhaps because in water we hold our breath long, right ?

Thanks

To begin, crabs are members of the phylum arthropoda, which includes spiders, insects and other crustaceans like shrimp, lobster, isopods (subphylum crustacea). Also they have only ten appendages in total, one pair for manipulation of objects (chelipeds) and the other four pair for walking. They have rather complicated mouth parts with small pincers and mandible types. Variations in the set-up of the mouth parts across all the crab species allow them to eat many different food types like biofilms on sand grain, vegetation like algae, or animals like molluscs, fish, etc.

Then how do you explain fish that *do* breath air *and* oxygen in water, i.e. some specialized fishes in the mangroves?

Are you talking about anabantoids? They have a specialised organ (the labrynth organ) for breathing air, that is seperate from the gills.

As far as I'm aware, lungs are capable of removing oxygen from water, but the problem is they can't do it anywhere near fast enough to sustain a person. The oxygen content of water is quite variable, but at best, still a small fraction of that in air. The lungs also aren't designed to move water in and out the same as they can with air, as water is a lot more dense. This means that once water has entered the lungs, most of it will not leave by normal breathing, and so it cannot be replenished with oxygenated water.
Gills work by a different principle to lungs. Unlike lungs, gills have an inlet and an outlet for water, instead of it having to enter and leave through the same route. This allows water to pass through the lamellae (efficient gas absorbing parts. I'm not going to go into detail about how they work) almost constantly, increasing the amount of time that oxygen is being absorbed. Even if humans had gills, they probably would not be able to survive underwater, as they are warm blooded and require more oxygen than cold blooded creatures such as fish.

To speak to the "humans returning to water" question, I would think that it really goes back to how long we have, as an evolutionary line, been "out of the water". If you believe in evolution then we humans evolved from much simpler creatures a long time ago and this established our path to land-based life. It's not like an even remotely human creature crawled out of the oceans and decided to give dry land a try. Our line has been firmly entrenched ( no pun intended) on land for quite sometime and while it may not necessarily be an evolutionary step backwards for us to go return to the water, it sure would be one heck of a step sideways.